This invention relates to an article having a superalloy protective coating and, more particularly, to such an article that resists the formation of a secondary reaction zone.
A protective coating system may be used to protect the components of a gas turbine engine that are subjected to the highest temperatures. The protective coating system includes a protective coating that is deposited upon a superalloy substrate, and optionally a ceramic thermal barrier coating that is deposited upon the protective coating. The protective coating inhibits oxidation and corrosion of the substrate, and also bonds the ceramic thermal barrier coating, where present, to the substrate. The thermal barrier coating acts as a thermal insulator against the heat of the hot combustion gas. Where there is no thermal barrier coating present, the protective coating is termed an xe2x80x9cenvironmental coatingxe2x80x9d; where there is a thermal barrier coating present, the protective coating is termed a xe2x80x9cbond coatxe2x80x9d. Examples of protective coatings include diffusion aluminides such as platinum aluminides and overlay coatings such as MCrAlX coatings.
The protective coating is typically rich in aluminum. Upon oxidation, the protective coating forms an alumina protective scale on its outwardly facing surface. Other oxides such as chromia may also be present, if the protective coating is also rich in the elements that form these other oxides. The protective scale inhibits further oxidation of the underlying structure and serves as a barrier against corrosion.
However, it has been observed that the presence of aluminum and other elements in the protective coating may in some cases lead to degradation of the underlying substrate after exposure of the structure to the operating temperatures experienced in gas turbine engines. See, for example, U.S. Pat. Nos. 5,334,263 and 5,935,353, whose disclosures are incorporated by reference in their entireties. Aluminum diffusing into the substrate may result in the formation of a secondary reaction zone (SRZ) that embrittles and otherwise degrades the properties of the substrate in the SRZ. The SRZ typically extends from about 0.002 to about 0.010 inches into the substrate, but even this depth of property degradation is highly significant in thin-walled components. Internally cooled gas turbine blades may have wall thicknesses as small as about 0.020 inch, so the SRZ formation may degrade the properties over as much as half of the thickness of the wall.
The approaches to addressing the SRZ problem disclosed in the ""263, ""353, and other patents are all operable in their particular circumstances. However, in other circumstances they may not be successful in avoiding the formation of an SRZ. There remains a need for solutions to the SRZ problem in these other circumstances. The present invention fulfills this need, and further provides related advantages.
The present approach provides an article protected by a protective coating, and a fabrication method for preparing such an article. The protective coating is highly oxidation- and corrosion resistant. The protective coating may be used as an environmental coating or as a bond coat for a thermal barrier coating. The protective coating may be applied by known techniques. The protected article has little or no susceptibility to the formation of a secondary reaction zone when the substrate and the protective coating interdiffuse.
In one embodiment, an article protected by a protective coating comprises a substrate having a substrate surface. The substrate is made of a first nickel-base superalloy substrate material that is susceptible to the formation of a secondary reaction zone when overlaid by a diffusion aluminide coating or an aluminide overlay coating. A protective coating overlies the substrate surface. The protective coating includes a deposited coating having a protective coating outer surface. The protective coating comprises a second nickel-base superalloy different from the first nickel-base superalloy, and which does not produce a secondary reaction zone when interdiffused with the first nickel-base superalloy. A ceramic thermal barrier coating may overlie the protective coating outer surface. Preferably, the article is a component of a gas turbine engine such as a turbine blade, a turbine vane, or a turbine shroud.
The deposited coating preferably has a composition, in weight percent, of from about 7.4 to about 7.8 percent chromium, from about 5.3 to about 5.6 percent tantalum, from about 2.9 to about 3.3 percent cobalt, from about 7.6 to about 8.0 percent aluminum, from about 0.12 to about 0.18 percent hafnium, from about 0.5 to about 0.6 percent silicon, from about 3.7 to about 4.0 percent tungsten, from about 1.5 to about 1.8 percent rhenium, from about 0.01 to about 0.03 percent carbon, from about 0.01 to about 0.02 percent boron, balance nickel and incidental impurities. A preferred nominal composition, in weight percent, is about 3.1 percent cobalt, about 7.6 percent chromium, about 7.8 percent aluminum, about 5.45 percent tantalum, about 3.85 percent tungsten, about 1.65 percent rhenium, about 0.02 percent carbon, about 0.016 percent hafnium, about 0.015 percent boron, about 0.5 percent silicon, balance nickel and incidental impurities.
In one specific and preferred embodiment, the substrate has a composition of from about 0.4 to about 6.5 percent ruthenium, from about 4.5 to about 5.75 percent rhenium, from about 5.8 to about 10.7 percent tantalum, from about 4.25 to about 17.0 percent cobalt, from 0 to about 0.15 percent hafnium, from 0 to about 0.06 percent carbon, from 0 to about 0.01 percent boron, from 0 to about 0.02 percent yttrium, from about 0.9 to about 2.0 percent molybdenum, from about 1.25 to about 6.0 percent chromium, from 0 to about 1.0 percent niobium, from about 5.0 to about 6.6 percent aluminum, from 0 to about 1.0 percent titanium, from about 3.0 to about 7.5 percent tungsten, wherein the sum of molybdenum plus chromium plus niobium is from about 2.15 to about 9.0 percent, and wherein the sum of aluminum plus titanium plus tungsten is from about 8.0 to about 15.1 percent, balance nickel and incidental impurities. This composition is subject to the formation of the SRZ when protected by diffusion aluminide or aluminide overlay coatings.
In another embodiment, an article protected by a protective coating comprises a substrate, preferably a nickel-base superalloy, having a substrate surface, and a deposited coating overlying the substrate surface. The deposited coating has the nominal composition set forth above. The other features set forth above may be used with this embodiment, where compatible.
A method of fabricating an article protected by a protective coating comprises the steps of providing a substrate having a substrate surface, and applying a protective coating, including a deposited coating, to the substrate surface. The substrate and the deposited coating are as described above. The deposited coating is preferably deposited by a spray process or a physical vapor deposition process. The deposited coating may thereafter be controllably oxidized, and/or a ceramic thermal barrier coating may be deposited overlying the protective coating outer surface.
In the present approach, the deposited coating is typically not highly alloyed with aluminum and other elements that cause the formation of an SRZ in otherwise-susceptible substrate alloys. As discussed in the ""263, ""353, and other patents, the susceptibility of a particular substrate alloy is determined in a straightforward manner by depositing a candidate protective-coating material onto the surface of the substrate alloy, heating this diffusion couple to cause interdiffusion to occur, and observing whether an SRZ forms. In the present approach, there is no SRZ formation under these circumstances, yet the protective coating has good oxidation and corrosion resistance and adheres well to the substrate in its role as an environmental coating. The protective coating performs these functions and also may serve as a bond coat for an overlying thermal barrier coating.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.